The tires are filled with air particles, whose behaviour is governed by what is known as the kinetic molecular theory. In simple terms, during the summer where temperatures are (assumingly) higher, the air particles have more energy (high temp. = higher average kinetic energy).With higher energy, they are moving about inside the tire with higher speed and collisions are more forceful (and common). Thus, these particles tend to occupy more space, and so the tire is forced to expand, eventually bursting.
